Somebody told me once that it was because they are trying to breathe. The water forces the air out of the soil and they come to the surface to try to find oxygen.
The rain water gets in their ears and screws up their equilibrium which causes them to dig up instead of down. Or maybe they hear the rain and come up to see what's going on up there. One of those I'm sure.
Earthworms can survive for several weeks under water providing there is sufficient oxygen in the water to support them. They surface as a response to high relative humidity after rain They can move around safely without dehydrating. One theory is that surfacing before and during heavy rains is an evolutionary behavioral adaptation enabling them to be dispersed.
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